Acoustic scanning probe microscopy methods, such as ultrasonic atomic force microscopy, form a class of imaging and research methods that allows the accurate imaging of subsurface features in a sample. The accuracy and size scale with which this may be performed, are however determined by a number of factors, such as the acoustics underlying the methods performed and the system characteristics (e.g. of the probe and probe tip).
Ultrasonic force microscopy (UFM), is for example performed by applying an ultrasonic signal to the sample (i.e. at MHz range) and modulating the ultrasonic wave with a modulation frequency of approximately the cantilever resonance frequency (i.e. at kHz range). By sensing the output signal at the modulation frequency and analyzing the amplitude and/or phase, subsurface structures can be imaged. This is due to the fact that the high frequency ultrasonic signal is perturbed by the subsurface structures. Information on the subsurface structures is conveyed via these perturbations and becomes measureable in the deflection of the probe tip, i.e. the output sensor signal at the cantilever frequency. However, to obtain an image with optimal contrast—i.e. a good signal to noise ratio—the device parameter settings must be well tuned, which is a difficult process as many parameters depend on each other. Moreover, the parameter settings that provide optimal amplitude contrast, do not necessarily provide optimal phase contrast.
The technology for subsurface imaging may be advantageously applied in industrial settings. For example, in semiconductor fabrication processes, the technology may be applied to perform defect inspection, e.g. of overlay error, alignment error, mask defects or blanks, all causing device defects. As semiconductor device become smaller and smaller, the need for industrialization of imaging techniques such as acoustic scanning probe microscopy methods continues to increase.